This application claims priority to co-pending United Kingdom Patent Application No. 9825890.8
The present invention is concerned with the limitation of brake pedal travel within the context of electro-hydraulic (EHB) braking systems during ABS braking.
A typical EHB system for a vehicle comprises a brake pedal, respective braking devices connected to the vehicle wheels and which are capable of being brought into communication with electronically controlled proportional control valves in order to apply hydraulic fluid under pressure to the braking devices, a hydraulic pump driven by an electric motor, and a high pressure hydraulic pressure accumulator fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking devices via the proportional control valves in order to apply hydraulic fluid under pressure to the braking devices in so called xe2x80x9cbrake by wirexe2x80x9d mode in proportion to the driver""s braking demand as sensed at the brake pedal. The EHB system is controlled by an electronic controller (ECU).
In order to make the EHB system xe2x80x9cfeelxe2x80x9d like a conventional braking system in a so-called xe2x80x9cpush-throughxe2x80x9d mode of braking in the event of failure of the brake-by-wire mode, a travel simulator is also provided which is connected hydraulically to a master cylinder coupled to the brake pedal and which allows, by increasing the volume under pressure, the brake pedal to be depressed to an extent comparable with that of conventional systems. In some known systems, the travel simulator may be isolated during this mode of braking by the closure of an electrically operated valve in order to reduce the pedal travel required to apply the desired braking pressure.
Known systems have the problem that, with brake-by-wire not operative due to failure, when the pressure applied to the wheel brakes ceases to be controlled as a function of master-cylinder pressure and pedal travel, ie when vehicle dynamic intervention such as ABS is active, the driver may still depress the brake pedal beyond the point at which travel and master cylinder pressure correspond to the applied braking pressure. This makes it difficult for the driver to modulate braking demand around the level at which ABS is activated.
Furthermore, when road conditions change and adhesion improves such that ABS is no longer required, the applied braking pressure reverts to its dependence upon master-cylinder pressure and pedal travel which, in the worst case, may be at their limit due to the driver""s wish to decelerate at a greater rate. The sudden increase in demand results in excessive deceleration and further difficulty in modulating braking pressure smoothly.
It is an object of the present invention to provide a braking system which alleviates the abovementioned problems with known systems without substantially adding to the complexity and/or cost of the braking system and without compromising safety.
In accordance with the present invention, the travel simulator isolation valve is arranged to be closed when the ABS mode is active.
During ABS activity, the brake pedal will then be unable to be pushed beyond a position corresponding to a wheel-locking pressure, thereby making it easier for the driver to modulate the brake pressure and avoid excessive deceleration if the road adhesion improves.
Preferably, closure of the travel simulator isolation valve is arranged to be delayed until at least two of the vehicle wheels are under ABS control.
Advantageously, when the travel simulator isolation valve first closes, the master cylinder pressure is arranged to be memorized, the memorized value being used subsequently to control re-opening of the valve.
Preferably, the valve is arranged to remain closed until either the master cylinder pressure falls below the memorized pressure, or until less than two wheels remain under ABS control.
The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings.